Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1711s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1711
Physics
[5465] Planetary Sciences: Solid Surface Planets / Rings And Dust, [6015] Planetary Sciences: Comets And Small Bodies / Dust, [6022] Planetary Sciences: Comets And Small Bodies / Impact Phenomena, [6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
Several sets of observations have provided evidence for the presence of greater abundances of dust in the lunar exosphere than can be explained by the tenuous impact-generated ejecta cloud anticipated to extend out for many thousands of km. Two modes for the electrostatic transport of dust have been proposed: levitation and lofting. While these mechanisms seem to provide reasonable explanations of how dust is transported at altitudes of ~1 m and ~1 km, respectively, they cannot explain the high altitude dust abundances that appear to exist at tens of km; e.g., as inferred from the "excess brightness", or lunar horizon glow (LHG), measured in Apollo coronal photographs. The Apollo 15 photographic sequence taken above the sunrise terminator indicated submicron dust concentrations at the surface of ~0.01-0.1 cm-3 and scale heights of ~10-15 km. In contrast, the evidence for LHG in the two sequences above the sunset terminator was tentative at best. Based on this limited set of sequences, there did not appear to be any correlation between high altitude LHG and either solar wind or solar UV conditions, which suggested that electrostatic transport was not responsible. However, there was an indication of a sunrise-sunset asymmetry, similar to that found in the in-situ measurements of charged dust by the Apollo 17 LEAM experiment. Due to the orbital motion of the Earth-Moon system about the Sun, there is a sunrise-sunset (leading-trailing) asymmetry in the sporadic meteoroid flux incident at the lunar surface. This lead to the suggestion that perhaps meteoroid impacts initiate a "saltation-like" process in which the ejecta not escaping from the Moon returns to the surface and ejects yet more dust. With each cycle in this cascade, the ejected dust decreases in velocity, but tends to increase in abundance - hence "saltation-like". This would likely result in higher concentrations and shorter scale heights. (Note that Apollo 15 also coincided with the Southern Delta Aquiriid meteoroid stream.) Based on a combination of empirical and theoretical models, an attempt is made to quantitatively assess the "saltation" hypothesis and determine whether it could explain the inferred submicron exospheric dust abundances at tens of km in altitude.
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